WebUI: Red 'X' displayed on a valid dependent attribute when saving a work item in Web UI

Using RTC 3.X or 4.0 GA
I have set-up my Project Area so as to have Priority attribute depends on Severity.
Severity = Major --> Priority = High
and so on...

Use case:
create a work item
set Severity = Major --> Priority = High
save
set severity = minor
a Red X is displayed next to Priority - value not applicable - that's the expected behavior - OK
change Priority to Low (which is the correct mapped value)
Red X disappear: OK
Save the work item.
The Red X - value not applicable is here again.

Is this a bug?

Note that refreshing the page clears this issue.